Method of applying fused silica coating to a substrate

ABSTRACT

DISCLOSED A METHOD OF APPLYING AN AMORPHOUS FUSED SILICA COATING TO A SUBSTRATE OF REFRACTORY NON-METALLIC MATERIAL. HIGH TEMPERATURE ARC PLASMA FROM AN ARC PLSMA SPRAY GUN IS PROJECTED ONTO THE SUBSTRATE SURFACE; AND PARTICLES OF A MATERIAL CONSISTING ESSENTIALLY OF CRYSTALLINE SILICA ARE FED INTO THE PLASMA, WHERE THEY ARE MELTED AND ENTRAINED IN THE PLASMA. THE GUN IS HELD CLOSE ENOUGH TO THE SUBSTRATE TO EFFECT MELTING OF THE SURFACE OF THE SUBSTRATE, OR THE PREVIOUSLY-APPLIED MATERIAL, IN THE REGION WHERE THE MOLTEN DROPS OF SILICA ENTRAINED IN THE PLASMA ARE IMPINGING AGAINST SAID SURFACE.   D R A W I N G

June 1974 c, BAlLEY ETAL 3,814,620

METHOD APPLYING A FUSED SILICA COATING TO A SUBSTRATE Filed Oct. 18, 1971 United States PatentOflice 3,814,620 METHOD OF APPLYING A FUSED SILICA COATING TO A SUBSTRATE Cecil Bailey, Woodlyn, and Walter C. Balchunas, Linfield, Pa., and John E. Zlupko, Delran, N.J., assignors to General Electric Company Filed Oct. 18, 1971, Ser. No. 190,190 Int. Cl. B05b 7/22; H01h 33/08 US. Cl. 11793.1 PF 4 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND This invention relates to a method of applying a fused silica coating to a substrate of refractory non-metallic material and, more particularly, relates to a method of applying such a coating by are plasma spraying.

For certain high temperature applications, e.g., an arc chute for an electric circuit breaker, fused silica is a nearly ideal material due to its exceptionally high resistance to thermal shock, its good electrical insulating properties, even when hot, and its ability to withstand exposure to a high current are with little erosion and with little evolution of gases. Eiforts have been made to apply the fused silica coating to a substrate by an arc plasma spraying process, but those prior efforts of which we are aware have not been very successful because the particles impinging against the substrate either have not formed a coating or have not adhered sufficiently firmly to the substrate. In addition, these efforts have en countered spraying difiiculties, such as the formation of fibers instead of molten droplets, which fibers have clogged the plasma spray gun.

SUMMARY An object of our invention is to provide an arc plasma spraying process for applying to a substrate of nonmetallic material a fused silica coating which tenaciously adheres to the substrate.

In carrying out our invention in one form, we provide an arc plasma spray gun in which we develop a high temperature are plasma that is projected onto a surface of refractory non-metallic material. We feed into this arc plasma particles of a material consisting essentially of crystalline silica, which particles are melted by the high temperature are plasma and entrained in said are plasma. The gun is held close enough to the substrate during the time are plasma is being projected thereon to eifect melting of the surface of the substrate or the previouslyapplied material in the region where said molten particles of silica are impinging against said surface.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of the invention, reference may be had to the following description taken in conjunction with the accompanying drawing, wherein:

FIG. 1 is a diagrammatic view of the arc plasma spraying process being applied to a portion of an arc chute 3,814,620 Patented June 4, 1974 of an electric circuit breaker. The are chute portion is shown in section.

FIG. 2 is a plan view showing the arc chute and the pattern followed by the arc plastma spray gun during a spraying operation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a base 10 of refractory non-metallic material to which it is desired to apply a coating of amorphous fused silica. In the illustrated example, the base 10 is a portion of an arc chute for a high power electric circuit breaker, such as shown, for example, in application Ser. No. 162,431 Frind et al., now Pat. No. 3,735,074, filed July 14, 1971, and assigned to the assignee of the present invention. The base 10 comprises a body 11 and relatively long fins 12 projecting therefrom in spaced apart relationship. Between these long fins are relatively short fins 14. The fins 12 and 14 are elongated elements extending generally parallel to each other, as can be seen in FIG. 2.

In one embodiment of the invention, this are chute portion 10 is formed of a refractory material made by reacting concentrated orthophosphoric acid and chrysotile asbestos, as is disclosed, for example, in US. Pats. 2,366,485Brink et a1. and 2,704,381-Ne1son. Further by way of example and not limitation, a preferred composition of such material also includes a zircon filler in an amount of about 60 percent by weight of the composition. This composition is capable of being hot molded and subsequently machined into intricate forms of good mechanical strength.

We apply the fused silica coating by relying upon an arc plasma spray gun of a conventional form. This spray gun comprises a tubular housing 20 which includes an arcing chamber 21 and an exhaust nozzle 22 leading from the arcing chamber to the exterior. Within the arcing chamber, there is an arcing electrode 24, which serves as a cathode, that is suitably electrically insulated from housing 20, which serves as an anode. An electric are 26 carrying direct current is formed between the cathodic electrode 24 and the anodic housing 20, and a suitable gas such as nitrogen is fed into the arcing region, as indicated by the arrows 27. This gas is converted into a hot arc plasma, by the high temperature of the are, and this plasma is discharged from the gun via the exhaust nozzle 22. The discharge from the nozzle may be thought of as comprising a plasma flame 30 and a spray stream 32.

The housing 20 contains a feed passage 34 which exhausts into the nozzle flow passage. Through this feed passage 34, spray powder suspended in a suitable carrier gas, e.g., nitrogen or argon, is fed into the plasma flame. Upon entering the plasma flame, the particles that constitute the powder are melted by the high temperature of the plasma flame, and these molten particles are entrained in the plasma stream.

The plasma stream containing these molten particles is projected from nozzle 22 onto the upper surface of the base 10. Upon striking the base or previously-applied spray material, the particles flatten and soon freeze, thus forming the desired coating.

As pointed out hereinabove, it is desired to apply an amorphous fused silica coating to the base 10. Prior efforts to apply a fused silica coating by are plasma spraying have not been very successful either because the particles impinging against the substrate have not formed a coating or have not adhered sufiiciently firmly to the substrate or because of spraying difliculties, such as clogging of the gun and fiber-formation, have been encountered. Many of these prior efforts have relied upon powdered fused silica as the source material which is fed into the plasma stream, and this we have found is one of the main sources of the above problems. Instead of fused silica, we use crystalline silica as our source material. This crystalline silica, in the form of quartz sand, is suspended in a suitable gaseous carrier, such as nitrogen or argon, and fed in this suspended condition through the feed passage 34 into the plasma stream. The high temperature of the plasma stream melts the particles of crystalline silica and converts them into molten droplets of fused silica that are entrained in the plasma stream. The coating that results from freezing of these particles upon their impacting against the base is essentally pure fused silica, containing essentially no crystalline silica. Microscopic examinations of the coating have shown no more than a few percent of crystalline silica in the coating.

In order to form a strong bond between the fused silica and the substrate, it is important that the substrate be heated by the hot plasma flame to a temperature that melts its surface in the region where the molten particles of fused silica are impacting against it. Unless this localized melting occurs, the bond between the coating and substrate is poor. The surface melting reduces the sharpness of the interface between the coating and the substrate and provides for more of afusion of the two materials in this region. The hot plasma flame is believed to maintain the previously-applied silica in a molten state during the time that additional molten silica particles are impinging thereagainst, thereby fusing together all of the superimposed silica particles in an amorphous structure of fused silica.

"For effecting this melting of the surface of the substrate, we hold the nozzle 22 of gun so close to the surface of the substrate that the substrate surface is located either within the plasma flame or closely adjacent the end of the plasma flame 30. In one form of the invention, this distance between the nozzle and the substrate is one to two inches.

A fused silica coating applied in the above described manner has excellent thermal shock resistance. As an indication of this, we have tested sample specimens (2 inches by 4 inches) coated in this manner by repeatedly Y passing across their fused-silica coated surface an oxyacetylene flame until the surface has reached red heat, after which the specimen has been plunged into a bath of water at room temperature. Despite this harsh treatment, the coating has remained undamaged and has retained its tenacious bond to the substrate. As another indication of the excellent thermal shock resistance of the fused silica coating, arc chutes coated in this manner have been exposed to high current-arcs tens of thousands of amperes in magnitude with little or no spalling or break-off of the coating. Specific examples of such are chutes are disclosed and claimed in the aforesaid Frind et al. application S.N. 161,431.

In applying the coating to the finned base 10 of FIG. 1, we move the arc plasma gun perpendicular to the longitudinal dimension of the fins 12, 14, as indicated by the arrow 40 in FIG. 1. The plan view of FIG. 2 illustrates by dotted lines 40a, 40b, and 400 the parallel paths followed by the gun during three successive passes, each pass being perpendicular to the length of the fins 12 and 14. The path of the first pass is represented by dotted line 4011; that of the second by 40b; and that of the third by 400. Each pass of the gun follows a path parallel to and closely adjacent the preceding pass, so that the coating resulting from each pass overlaps slightly that resulting from the preceding pass. This perpendicular movement reduces the amount of energy that is supplied to any one fin during a given pass of the gun, as com pared to the energy that would be supplied if the gun was moved longitudinally of the fin being coated during each pass. This reduced energy appreciably reduces the tendency of the fin to warp and crack under the heat-induced stresses. The gun is moved a short distance to the left on each successive pass until the entire area which it is desired to coat is sprayed.

In one embodiment of the invention, where the base is of phospho-asbestos material, we utilize the plasma spraying operation both to flame treat the base material and to apply the desired coating. In this embodiment, the heat of the plasma flame during the spraying process is relied upon to partially degas and to partially remove absorbed and chemically-bound water from the substrate in the region adjacent to its exposed surface.

In another embodiment of the invention, we achieve the desired flame treating by first flame treating the substrate with the arc plasma discharge without spraying and then arc plasma spraying the substrate following completion of the flame treatment. In this second embodiment of the invention, it is not necessary to limit the speed of movement of the gun during spraying by any requirement for degassing of moisture removal; but it is necessary that movement of the gun be sufficiently slow to allow surface melting during arc plasma spraying. In this second embodiment, as applied to the finned arc chute, both the flame treatment and the spraying operations are performed by passing the gun along parallel paths perpendicular to the length of the fins, thereby greatly reducing the tendency of the fins to warp and crack, as previously explained. While flame treatment of the fins has heretofore been performed by moving a plasma gun along such perpendicular paths, this was not a spraying operation and we are not aware of any thermal spraying operation that has been performed by using such perpendicular paths.

Although we have described our process particularly in connection with the coating of a substrate made of a zircon filled phospho-asbestos material, it is to be understood that this has been done by way of example and not limitation. Our process can also be used in coating many other refractory non-metallic materials, e.g., phospho-asbestos material without the zircon filler, portland cementasbestos such as disclosed in US. Pat. 2,270,723-Boehne, and glass-bonded mica.

It is also to be understood that, in its broader aspects, our invention is not limited to use with a base having fins. In this respect, we can use our process for applying a fused silica coating to a flat or a curved surface containing no fins or the like.

While we have shown and described particular embodiments of our invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from our invention in its broader aspects; and we, therefore intend herein to cover all such changes and modifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In the method of arc plasma spray coating a fused silica coating onto a phospho-asbestos substrate having a surface with a series of generally parallel elongated fins projecting from said surface, said method comprising arc plasma spraying crystalline silica which during spraying melts and deposits as fused silica, the crystalline silica being fed into a high temperature arc plasma developed within an arc plasma spray gun and exhausted through a nozzle of said gun, the improvement comprising moving the arc plasma spray gun repetitively along said surface in a direction generally perpendicular to the length of said elongated fins to limit heat build-up in said fins thereby to substantially prevent warping said fins.

2. In the method of claim 1, the further improvement of holding the nozzle of said spray gun close enough to said substrate during the times that arc plasma containing molten silica particles is being projected thereon to effect melting of the surface that said molten particles are im pinging against.

3. The method of claim 1 in which said are plasma spray gun is moved across the surface of said substrate in successive passes following generally parallel paths that are sufficiently close together that the spray coating from one pass overlaps with that of the immediately-preceding pass.

4. The method of claim 1 in which said substrate is pretreated prior to the spraying step by passing a high temperature flame over the surface to partially remove gases and water from the portion of said substrate adiacent said surface.

References Cited UNITED STATES PATENTS 3,649,406 3/1972 McNish 117--126 AF 6 FOREIGN PATENTS 345,653 2/1920 Germany 117l05.2 343,664- 11/1921 Germany.

US. Cl. X.R.

11754, 126 AF; 200-144 C 

